How Stratigraphy Is Used
In the case of societies that have left no written histories, the excavation and recording of strata, features and artifacts often provides the only method of learning about those societies. Even when recorded histories exist, stratigraphic investigations can provide an excellent complement to what is already known.
According to the law of superposition, in a given series of layers, as originally created, the upper layers are younger and the lower layers older because each layer presumably has been added to a pre-existing deposit. Based on this law, archeologists have been able to assign dates, in relative sequence, to stratified layers. The law of superposition is not infallible. Sites often contain strata that have been disturbed by natural processes, such as floods, and human activities, such as digging. In these instances, several original layers may be intermixed, and the artifacts contained within may be out of chronological sequence.
In stratigraphic excavations, deposits from a site are removed in reverse order to determine when they were made. Each deposit is assigned a number, and this number is appended to all objects, including artifacts, bones, and soil samples containing organic matter, found in the layer. Each layer provides a unique snapshot of a past culture, the environment in which it existed, and its relative period in time. Stratigraphic dating does not require the existence of artifacts, but their presence may facilitate dating the site in absolute time. Without such clues, it can be very difficult to date the layers; a deep layer of sand, for example, might have been deposited very quickly in the course of a sand storm, while another layer of the same thickness could have taken hundreds of years or longer to form.
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